Units and conventions xix
Astronomical unitsIn astronomy, distances are usually measured in parsecs and
megaparsecs instead of centimeters. They are related to centimeters via
1pc= 3 .26 light years= 3. 086 × 1018 cm, 1 Mpc= 106 pc.
The masses of galaxies and clusters of galaxies are expressed in terms of the mass
of the Sun,
M 1. 989 × 1033 g.Charge unitsWe use the Heaviside–Lorentz system for normalization of the ele-
mentary electric chargee.This system is adopted in most books on particle physics
and in these units the Coulomb force between two electrons separated by a distance
ris
F=e^2
4 πr^2.
The dimensionless fine structure constant isα≡e^2 / 4 π 1 / 137.SignatureThroughout the book, we will always use the signature(+,−,−,−)for
the metric, so that the Minkowski metric takes the formds^2 =dt^2 −dx^2 −dy^2 −
dz^2.